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Posted for Gary Casey <casey.gary@yahoo.com>:
Re: [LML] Prop and Glide considerations
December 18, 2009 8:18:44 AM MST
From:
"Colyn Case at earthlink" <colyncase@earthlink.net>
To:
"" <lml@lancaironline.net>
great post, Scotty. ....but I've never understood the physics of why a
windmilling prop has so much more drag than a stationary prop.
Here's a not-very-scientific explanation: The drag in each mode comes from
two different sources. With a stopped prop the blades are certainly stalled
and perform more like a flat plate. Not different than pushing a flat plate
equal in area to that of the blades. With a turning prop the drag is more
like the induced drag of a wing. The prop is converting energy from the air
into the power required to spin the engine at that rpm. So the hp loss is
from the friction hp of the engine, which goes up at about the square of the
engine rpm. Going to a coarser pitch doesn't change the drag coefficient of
the prop much - it just reduces the power required to drive the engine. That
might explain why a stopped prop, as shown in a previous post, doesn't always
produce lower drag. It would depend on speed, engine friction
characteristics, number of prop blades, etc. I vote for forgetting about
stopping the prop - there are enough distractions without that.
And Bill mentioned an interesting fact - at the coarse pitch setting,
simulating and engine out by just going to idle may not be realistic. Most
engines with the throttle at idle quit running at about 1500 rpm, more or
less. Above that rpm the engine ceases to fire and the torque is the same as
if the fuel were shut off (all modern car engines have fuel cutoffs if the
speed is high and the throttle closed). With the prop in the fine pitch (max
rpm) setting closing the throttle will likely produce a good simulation of
engine-out performance. However, with the prop control pulled back the engine
will likely drop to below 1500 rpm and in that case it will still be running
and producing some torque, reducing the power that has to be supplied by the
prop. Testing with a closed throttle will produce an overly optimistic glide
ratio. If someone would report on the glide ratio of an ES with the engine
shut off I would be very interested - I'm not going to
do the test myself, thank you.
Gary
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